replacing coolant is simple, your thought on how to do it are pretty much correct.
would also remove the hose on the side of the water pump, above the thermostat so that you get the last bit out that otherwise wouldn't.
then you need to decide what to do to change the thermostat, you have two choices.... (yes, yours has had it)
1) remove alternator, then alternator mount, water pump, then take flange off and replace thermostat and o-ring. no need to jack the car up.... but watch that coolant does not run onto the cambelt.
ORR:
2) undo p/s pump (don't disconnect hydraulic line), this will allow access to the thermostat housing and you can get it off, you will need to jack it up for this most likely, put an axle stand under the front cross member to secure it....
btw you don't have to put the heater to hot in a mk3.... the matrix is always in the flow path.... (unlike in a Ford KA for example where you have a valve before the matrix)
no need generally to replace the expansion tank cap, unless you still got a black one (they are not brilliant! and 92 model is very likely to still have that), then buy a blue one...
you can clean expansion tank, just take it out, plug up the holes... nick a few dental cleaner pills from gran and put them in there with hot water, scrub with a toothbrush the flush.... OR pick up one from ebay if you're that bothered.
thermostat you need is 87 degrees version. the lower one is for some of the smaller engines or vr6.
if you got a choice go for a "behr" make, they are the best around. Had a quick look on euro car parts...
one of them is with seal, one without. buy the one with the seal - the original one from 92 might not seal anymore and you don't wanna redo whole procedure because of a fecking o-ring

ECP don't make it easy. will have a quick check which i put in my 2.0 8v... thats the same as for 1.8. => either should work... or GSF part no. 176VG0070
for flushing, undo both hoses on heater matrix.... garden hose with gardena/hozelock connector and flush it through... same for radiator.... then you can just buy a cooling system cleaner from ebay... hafords, ecp, gsf....
when you've flushed the system sufficiently replace with 50% g12++ and 50% water (tap's fine unless you're in an area with very hard water, then mix DI or distilled with tap).
door mirror thing, which colour is the wire? don't know by hard anymore which is which but can check.
i used two inline crimp connectors and a short piece of wire (same dia) and just cut wire and crimped it all up.....
btw, where are you from. if you're local to me can give you a hand.